I can't move and scale a picture that I want to set as a wallpaper on the ios7. Any idea how to zoom down?

Ditto. Seems to be a bug. On my iPhone 4S and iPad2, only shows the center portion of a photo, only allows horizontal scrolling, does not allow resizing, and iPad2 tends to freeze-up when trying to adjust the photo.

I'm struggling with it myself

i know how to fix this!

Screenshot the picture you want to set as your background, then set the screenshot as your background. It will be zoomed in a little, but it's better than just turning the parallax effect on.